Rayleigh-Bénard（R-B）对流广泛存在于日常生活及工业生产中，由于在这种简单的物理系统中会孕育出及其复杂多变的流动结构，因此，R-B对流的非平衡热动力学特征一直是国际前沿研究热点之一。相比于常规流体，具有密度极值流体的R-B对流的非平衡流动结构更加丰富、流型演化过程更加复杂，而这方面的研究还很不够。本项目即以具有密度极值流体层的R-B对流为研究对象，采用实验测量、数值模拟和理论分析相结合的方法，研究具有密度极值流体R-B对流的复杂特性，探索可能存在的流动结构，认知流型演变过程的物理机制，分析液层几何特性、流体密度极值特性、Rayleigh数和Prandtl数等对流动结构及其演变过程的影响，获取发生R-B对流的临界条件、流动结构稳定性图和流型分岔图谱。研究内容对于发展复杂流动非平衡热动力学理论、拓展R-B对流研究领域具有重要的科学意义和学术价值。

Rayleigh-Bénard convection driven by the buoyancy in the horizontal fluid layer heated below has received considerable attention not only for its existence in many daily and engineering fields, but also for its fundamental aspects as the rich dynamical behavior and the complex pattern formation determined by the nonlinear effect of this simple system, and therefore, it is one of the research hotspots all over the world. Comparing with the common fluid, the Rayleigh-Bénard convection of fluids with density extremum behaves extreme multiplicity of flow patterns and much more complex transition characteristics. Aiming at the complex Rayleigh-Bénard convection problem of fluids with density extremum, this project studies the flow pattern multiplicity and the transition mechanisms of the complex flow patterns driven by the buoyancy in the horizontal fluid layer heated below by combining the experimental observation, numerical simulation with theoretical analysis. First, various possible flow patterns are explored very carefully using and three-dimensional numerical simulation when the Rayleigh-Bénard convection appears. Then, the evolutions of the flow and temperature fields are discussed with the increase of the Rayleigh number. The physical mechanisms of the instability and flow pattern transition are seeked under the different conditions. Furthermore, effects of geometrical parameters, density extremum characteristic of the fluids, the Rayleigh number and Prandtl number on the flow pattern and transition are analysed. Finally, critical conditions of the flow transition, stability diagram and bifurcation diagram of the flow pattern are obtained by coupling three-dimensional numerical simulation with the linear stability theory. Research contents proposed in this project are of important scientific and academic values for developing the non-equilibrium thermo-dynamic theory of the complex flow and extending the research field of the Rayleigh-Bénard convection.